IGCSE · 5 · structured
Nuclear physics · Question 9
Nuclear physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- routine
- Marks
- 7
- Topics
- 1
- Answer
- Complete
A hospital uses a radioactive tracer to study a patient's kidneys. The tracer is a γ-emitter with a half-life of 6 hours and is injected into the bloodstream.
- (a)
Define Define half-life.
2 marks - (b)
Explain Explain why a γ-emitter is chosen for this use rather than an α-emitter.
3 marks - (c)
Suggest Suggest why an isotope with a half-life of 6 hours is more suitable than one with a half-life of 6 years.
2 marks
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(a)
Define Define half-life.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
the time taken for half the nuclei of the radioactive isotope in a sample to decay
- 2
or, equivalently, for the activity (count rate) of the sample to fall to half its original value
(b)
Explain Explain why a γ-emitter is chosen for this use rather than an α-emitter.
State the outcome first, then link cause to effect with the relevant physical principle. Each link in the reasoning should be explicit enough to earn its own marking point.
- 1
γ-rays are the most penetrating of the three, so they pass out through the body and can be detected outside it
- 2
α-particles would be absorbed within a few centimetres of tissue and never reach the detector
- 3
α is the most strongly ionising, so it would do far more damage to the surrounding cells
(c)
Suggest Suggest why an isotope with a half-life of 6 hours is more suitable than one with a half-life of 6 years.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
the activity falls quickly once the measurement is finished, so the patient's total exposure is small
- 2
an isotope with a half-life of years would keep irradiating the patient long after the scan, with no benefit
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